Method and apparatus for making nonintrusive noise and speech level measurements on voice calls

ABSTRACT

A digital signal processor is provided with the facility to measure accurately, and nonintrusively, the levels of noise and/or speech signals appearing on an in-service network connection.

FIELD OF THE INVENTION

This invention relates to the accurate measurement of noise and speechlevels in a transmission path, such as a telephone connection.

BACKGROUND OF THE INVENTION

The levels of noise and speech signals propagating through an in-servicetransmission path are used to determine the quality of the path. Forexample, the transmission quality of the path may be questionable if thelevel of the noise signals is high, or if the level of the speechsignals is low (weak). It can be appreciated, therefore, that anaccurate determination of the quality of a transmission path requires anaccurate measurement of the levels of noise and speech signals as theypropagate through the transmission path.

Prior measuring arrangements have been unable to achieve such accuracy,since they do not accurately measure the level of noise present in atransmission path. For example, one such prior measuring arrangementperforms multiple measurements over a respective 30 millisecond windowwhen noise signals are believed to be present on the transmission path.Noise signals are believed to be present when the level of the signal onthe path falls below a predetermined threshold. At that point, the priormeasuring arrangement measures the level of all signals that are presenton the transmission path within the 30 millisecond window. Thearrangement then determines an average noise value for the window. Whenthe prior measuring arrangement has completed a number of suchmeasurements, it then outputs, as the noise level of the transmissionpath, the average having the lowest value. This prior measuringarrangement does not provide an accurate noise level measurement becauseboth speech signals and noise signals could be present during the 30millisecond window, thereby biasing the resulting average.

U.S. Pat. No. 5,216,702 to the inventor hereof, the specification ofwhich is incorporated herein by reference, describes a method andapparatus for obtaining a nonintrusive measurement of noise and speechsignals present on in-service connection by first determining whethersamples of signals received from the connection correspond to noise orspeech. The noise and speech levels of the connection are thendetermined as a function of such samples. Specifically, individualaverage power levels are determined for succeeding groups of noisesignals that are present on the connection within a predetermined periodof time. An average power level is then determined for the succeedinggroups as a whole. When a predetermined number of such average powerlevels have been determined, the median of such average power levels isoutput as the noise measurement of the connection. A speech levelmeasurement is obtained in a similar manner.

The median of the average power levels was chosen as the output in themethod and apparatus of U.S. Pat. No. 5,216,702 because it provided anaccurate measurement of noise level during testing of the system underlaboratory conditions. Under laboratory conditions, the testingenvironment is generally well controlled because noise and speech areinjected into the system from recorded sources. However, when the systemwas tested in the field subsequent to the issuance of U.S. Pat. No.5,216,702, it was discovered that a median of the average power levelsdoes not provide an accurate measurement of noise. Specifically, asignificant amount of ambient background noise, such as a television ora baby, which is absent under laboratory conditions, exists in thefield. This background noise adds significant variability to the signalspresent on the transmission path. This variability due to backgroundnoise cannot be discriminated from the circuit noise present on thechannel, and thus a median of the average power levels tends tooverstate the actual circuit noise level of the channel.

SUMMARY OF THE INVENTION

Provided is a facility which obtains a nonintrusive measurement ofspeech and noise signals present on an in-service connection by firstdetermining whether samples of signals received from the connectioncorrespond to speech or noise, and then determining the noise and speechlevels of the connection as a function of such samples. In this way, adynamic noise measurement is obtained, in accord with an aspect of theinvention, by determining the minimum of a number of measurements of theaverage power levels of noise signals, in which the measurements areaccumulated over respective periods of time when it is known that noisesignals are present on the connection. The minimum of the average powerlevels under such a measuring scheme, once thought to underestimate thenoise measurement, has subsequently been discovered as providing a moreaccurate measurement of the noise present on the transmission path underreal world conditions. A speech level measurement is obtained in asimilar manner.

More specifically, an average power level is determined for succeedinggroups of noise signals that are present on the connection within apredetermined period of time. When a predetermined number of suchaverage power levels have been determined, then, in accord with anaspect of the invention, the minimum of such average power levels isoutputted to an output terminal as the noise measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the manner in which signal level measuring equipmentmay be connected to an in-service network connection for the purpose ofmeasuring the level of noise and/or speech signals propagating throughthe connection.

FIGS. 2-5 illustrate in flow chart form the program which implements thepresent invention in the digital signal processor of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates a conventional telephone connection establishedbetween station sets S1 and S2 via Central Offices (COs) 200 and 250 andinterexchange network 300. The way in which a telephone connection isestablished between telephone station sets is well-known and will not bediscussed herein. However, it is seen from the FIG. that such aconnection includes telephone lines 201, which connects station set S1to Central Office (CO) 200. At CO 200, a conventional hybrid arrangementconverts 2-wire telephone line 201 to a so-called 4-wire transmissionpath comprising paths 203 and 204. Paths 203 and 204 are then connectedthrough toll switch 305, intertoll connection 310 and toll switch 315 toCO 250 where another conventional hybrid arrangement converts paths 203and 204 into a 2-wire telephone line extending to station set S2.

As is well-known, interexchange network 300 may be of the type whichtransports speech signals via its associated intertoll network 310 indigital form. Accordingly, COs 200 and 250 include analog-to-digital anddigital-to-analog converters in the interface that they present atnetwork 300.

In order to gauge the quality of the transmission connection betweenstation sets S1 and S2, copies (samples) of the digital speech signalstraveling in the E and F directions along intertoll connection 310 aresupplied to transmission measurement arrangements 100 via leads 101 and102 are then presented to respective inputs of Digital Signal Processor(DSP) 115 via interface circuits 105 and 110, respectively. DSP 115,which may be, for example, the model DSP32 manufactured by LucentTechnologies, Inc., formerly the Systems and Technology Division ofAT&T, analyzes the digital samples that it receives from the E and Fdirections to determine if they represent speech or noise. Specifically,if particular digital samples represent a level which equals or exceedsa predetermined threshold, for example, a threshold having a level -42dBm, then DSP 115 operating in accord with the principles of theinvention concludes that the samples represent speech signals.

It is well-known that a speaker typically drops the level of his or hervoice when completing a response. Accordingly, the level of a speaker's"trailing" voice signals may be below the aforementioned threshold. Toaccount for that situation, DSP 115 is arranged in accord with an aspectof the present invention so that when it detects a speech signal thatequals or exceeds the threshold, it then considers all succeedingsamples that occur within a predetermined window--illustratively 200milliseconds--to be speech signals, even though the levels of thosesignals may be below the threshold, as will be explained below.

More specifically, DSP 115 is arranged to process a number of sampleseach second, e.g., 8000, for each of the E and F directions. Tofacilitate such processing, DSP 115 accumulates the squared values of agroup of samples, e.g., 16, as they are received. In a preferredembodiment of the invention, the samples are filtered with a 200 Hz HighPass filter before they are squared and summed. This filter removes lowfrequency noise from sources such as AC Power from the signal. DSP 115then determines an average power value for the group. If the averagepower equals or exceeds the aforementioned threshold of -42 dBm, thenDSP 115 considers the average power value and succeeding ones of suchvalues occurring within the aforementioned window to represent speech.As an aspect of the invention, DSP 115 is arranged to accumulate apredetermined number--illustratively 10,000--of such average powervalues that are speech for both the E and F directions. When it hasconcluded such processing, DSP 115 outputs via lead 103 the averagevalue of the 10,000 power values it accumulated for the E direction andfor the F direction. The outputted averages respectively represent thespeech level measurements for the E and F direction.

It can be appreciated that, during the measurement, speech signalstraveling in either the E and F direction could be an echo, which, ifincluded in the speech measurement, could bias the overall speechmeasurement that DSP 115 outputs to lead 103. To guard against such apossibility, DSP 115 is arranged, in accord with an aspect of theinvention, to determine if a group of speech samples that it receivesrepresents an echo. If DSP 115 finds that to be the case, it discardsthe group. In particular, if DSP 115 receives simultaneously a group ofspeech samples from both the E and F directions, then DSP 115 determinesif one of the groups represents an echo. If that is the case, then, asmentioned above, DSP 115 discards the average power level determined forthat group, as will be explained below.

In addition, if DSP 115 finds that the average power level of a group ofsamples received from either the E or F direction is below -42 dBm andis not within the aforementioned speech window, then DSP 115 considersthe group to be noise. If that is the case, then DSP 115 accumulates(sums) that average power level with the average power levels determinedfor succeeding groups of samples obtained from the same direction withina 200 millisecond window. However, if any one of such succeeding averagepower levels equals or exceeds -42 dBm, then DSP 115 discards theaccumulation. DSP 115 does so since a power level of -42 dBm most likelyrepresents speech and, therefore, would adversely effect the noisemeasurement represented by the accumulation, as will be explained below.

Turning now to FIGS. 2 through 4, the program which implements theprinciples of the invention in DSP 115 is shown in flow chart form.Specifically, when the program is entered at block 500 it clears anumber of flags, counters and accumulators (identified below) and thenproceeds to block 501. At block 501, the program accepts a sample of asignal traveling in the F direction over intertoll transmission path 310(FIG. 1), and then proceeds to block 502. At block 502, the programcalculates in a conventional manner the square of the voltage levelrepresented by the received sample. The program then adds the result ofthe calculation to a running sum (accumulation) designated FDSUM. Theprogram then increments a counter designated SUMCTR by a value of one.The program uses SUMCTR to determine when it has acquired a group of Nsamples from both the E and F directions, as will be seen below. In anillustrative embodiment of the invention, the value of N may be, forexample, 16. Similarly, at blocks 503 and 504, the program acquires asample of a signal traveling in the E direction and then determines thesquare of the voltage level of that sample. The program then adds thelatter result to a running sum designated EDSUM. The program thenproceeds to block 505.

As shown in FIG. 2, in a preferred embodiment of the invention, the Eand F direction samples are filtered with a C-message filter before theyare squared and summed. The C-message filter reflects the impact ofnoise at different frequencies on callers' opinion of quality.

At block 505, the program returns to block 501 if the value contained inSUMCTR indicates that it has not acquired N samples (e.g., 16 samples)for each of the two oppositely directed transmission paths. Otherwise,the program proceeds to block 506 where it clears (sets to zero) thecontents of SUMCTR and then proceeds to block 507. At block 507, theprogram calculates a value designated FDVAL (EDVAL), which isproportional to the average power level of the group of samples acquiredfrom the F (E) direction. The program then proceeds to block 508 whereit clears FDSUM and EDSUM and increments two counters respectivelydesignated FDHANGOVER and EDHANGOVER by a value of one. The program thenproceeds to block 509. (The purpose of the latter counters will be madeclear below. However, it suffices to say at this point that thosecounters represents a particular period of time which is defined hereinas being a "hangover" time.)

Program blocks 509, 510 and 518 through 520 represent a program modulewhich determines if the value of FDVAL determined at block 507represents speech. Program blocks 511, 512, and 521 through 523 performa similar function on EDVAL.

Specifically, at block 509, the program compares the value FDVAL with athreshold (TH) having a predetermined value--illustratively -42 dBm. Ifthe program finds that the values of FDVAL equals or exceeds the valueof TH, then it proceeds to block 518. At block 510, the program sets thecontents of counter FDHANGOVER to zero and sets a flag FDSTATE to equala value of one to indicate that the value of FDVAL represents speech.The program then proceeds to block 511.

As an aspect of the invention, the program classifies as speech signalssamples of signals acquired within 200 milliseconds of a group ofsamples whose power value, e.g., FDVAL, exceeds TH. Accordingly, theprogram uses a "hangover" time to include samples of weak speechsignals. The program implements the foregoing by incrementing hangovercounter FDHANGOVER by a value of one each time it passes through block508, in which, in accord with the aforementioned sample rate, a value ofone represents two milliseconds. In addition, the program clears counterFDHANGOVER at block 510 to ensure that the F-direction samples that areacquired within the next 200 milliseconds are classified as speechsignals.

In particular, when the program arrives a block 518, it compares thevalue represented by the contents of counter FDHANGOVER with apredetermined value M, e.g., 100, representing 200 milliseconds. Theprogram proceeds to block 519 if the comparison indicates that theformer value is less than the latter value. Otherwise, the programproceeds to block 520. At block 519, the program sets flag FDSTATE to avalue of one to indicate that the current value of FDVAL representsspeech. At block 520, the program sets the value of FDSTATE to zero toindicate that the value of FDVAL does not represent speech, since thatvalue is below TH and was derived from samples that were received afterthe expiration of the hangover time. The program then proceeds to block511 (FIG. 3).

As mentioned above, blocks 511, 512 and 521 through 523 perform asimilar function with respect to EDVAL. As such, when the programarrives at block 513, the value of flag EDSTATE will be set to a one orzero, indicating that the value of EDVAL represents speech or nonspeech(noise) signals, respectively.

Blocks 513, 514 and 524 represent a program module which determines if(a) speech signals were detected in one direction only, E or F, or bothdirections, or (b) nonspeech signals were detected in both the E and Fdirections. If speech is detected in both directions, then the program(blocks 526 and 527) determines, in accord with an aspect of theinvention, if either the E- or F-direction speech is an echo. If that isthe case, then the program retains the power level derived from the truespeech samples and discards the power level derived from the echo. Theprogram discards the latter power level, since it represents areflection of true speech signals. If the module determines that EDVALand FDVAL both represent speech, then the module retains both values andaccumulates them in a manner to be described below.

Specifically, the program at block 513 proceeds to block 514 if it findsFDSTATE is set to a value of one, which, as mentioned above, isindicative of speech. Otherwise, the program proceeds to block 524 whereit proceeds to block 531 if it finds that EDSTATE is set to a value ofzero, which, as mentioned above, is indicative of nonspeech signals(i.e., noise). Otherwise, the program proceeds to block 525.

At block 514 (525), the program clears two values, EDNOISE and FDNOISE,which it uses to accumulate the power level value determined for each ofa number of successive groups of samples that are found to representnoise and obtained from the E direction and/or F direction,respectively. The program also clears a counter, NSCTR, that it uses totrack the number of successive groups of E- and F-direction samples thatare found to be noise. The purpose of the EDNOISE and FDNOISE values andNSCTR counter will be made clear below. Following the foregoing, theprogram then proceeds to block 515.

At block 515, the program proceeds to block 610 if it finds that EDSTATEis also set to a value of one. Otherwise, the program proceeds to block516.

At block 516 (528) the program determines whether or not it hasprocessed a predetermined number--illustratively 10,000--groups ofF-direction (E-direction) speech samples. To make that determination,the program maintains a counter, FDCTR (EDCTR), which it incrementsfollowing the processing of a group of F-direction (E-direction) speechsamples. If the program finds that the value of FDCTR (EDCTR) equals orexceeds a value of 10,000 (noted in the FIG. as P) then it proceeds toblock 519 (530). Otherwise, the program proceeds directly to block 517(529). At block 519 (530), the program sets a flag designated FDDONE(EDDONE) to indicate that it has processed 10,000 groups of F-direction(E-direction) speech samples. The program then proceeds to block 517(529). At block 517 (529), the program adds the value of FDVAL (EDVAL)(i.e., the power level that the program derived from the current groupof samples obtained from the F (E) direction) to an accumulation,FDSPEECH (EDSPEECH), of such power levels. The program then incrementscounter FDCTR (EDCTR) by a value of one and then returns to block 501 torepeat the foregoing process.

The program reaches block 610 if speech is detected in both the E and Fdirections, in other words if EDVAL and FDVAL are both determined to bespeech. If this is the case, EDVAL and FDVAL are summed and storedtemporarily, and will be added to EDSPEECH and FDSPEECH, if they aredetermined to not be echos, either when the program is finished or whena predetermined number have been accumulated, for example 50. At block610, EDVAL and FDVAL are added to EDTSUM and FDTSUM, respectively. Inaddition, a counter DTCTR is incremented. At block 615, DTCTR is checkedto determine whether a predetermined number, for example 50, EDVAL andFDVAL values have been accumulated. If 50 have not been accumulated,then the program returns to block 501. Otherwise, as shown in block 620,FDTSUM is compared to two-times EDTSUM. If FDTSUM is greater thantwo-times EDTSUM, then EDTSUM is considered to represent echo signalsand the program proceeds to block 625, where EDTSUM is discarded, FDTSUMis added to FDSPEECH, and FDCTR is incremented by a value equal toDTCTR. Otherwise, the program proceeds to block 630, where EDTSUM iscompared to two-times FDTSUM. If EDTSUM is greater than two-timesFDTSUM, then FDTSUM is considered to represent echo signals and theprogram proceeds to block 635, where FDTSUM is discarded, EDTSUM isadded to EDSPEECH, and EDCTR is incremented by an amount equal to DTCTR.Otherwise, the program proceeds to block 640 where EDTSUM is added toEDSPEECH, FDTSUM is added to FDSPEECH, and EDCTR and FDCTR are bothincremented by a value equal to DTCTR. From blocks 625, 635 and 640, theprogram proceeds to block 645, where DTCTR, FDTSUM and EDTSUM are set tozero, and thereafter the block 536A.

At block 531, the program determines whether the value of a counter,APTR, is less than a predetermined value i, for example, a value of 11.APTR is initially set to a value of 1. The program uses theaforementioned NSCTR counter to track 100 consecutive FDVALs and EDVALsthat represent noise power levels, in which an NSCTR value of 100represents 200 milliseconds. In this way, the program stores the averagepower level of consecutive samples obtained within a 200 millisecondwindow from both the E and F transmission paths.

In particular, if the program finds that the value of APTR is less than11, then it proceeds to block 532. Otherwise, it proceeds to block 535.At block 532, the program (a) adds the values of FDVAL and EDVAL (whichrepresent noise) to accumulators FDNOISE and EDNOISE, respectively, and(b) increments counter NSCTR by a value of one. As stated above, in apreferred embodiment of the invention, the samples included in the noisesums (FDNOISE and EDNOISE) are filtered with a C-message filter beforethey are squared and summed (blocks 501-504). The C-message filterreflects the impact of noise at different frequencies on callers'opinion of quality. The program then proceeds to block 533, where itchecks to see if the value of NSCTR equals 100. If that is the case,then the program proceeds to block 550. Otherwise, it returns to block501. At block 550, the program sets counter NSCTR to zero. Then, asshown in blocks 555-575, the program determines whether the valuesFDNOISE/100 and EDNOISE/100 are the minimum of such values measured bythe program thus far. If a new minimum is found, that value is stored inthe appropriate one of EDNSMIN and FDNSMIN. The program then (a) clearsaccumulators FDNOISE and EDNOISE, (b) increments counter APTR by a valueof one and then (c) goes to block 536.

At block 535, the program sets a flag, NSDONE to indicate that it hasaccumulated sufficient data to calculate an accurate noise levelmeasurement for the E and F transmission paths and the program thenproceeds to block 532.

At blocks 536 through 538, the program determines if it has accumulatedthe required data to generate speech and noise measurements for both theE and F transmission paths and proceeds to block 539 if it finds that tobe the case. Otherwise, the program returns to block 501 via the NObranches of blocks 536A through 538.

The program arrives at block 539 as a result of having accumulated ineach accumulator FDSPEECH and EDSPEECH the average power values forapproximately 10,000 groups of respective speech samples and has storedin each of EDNSMIN and FDNSMIN the minimum power values relating tonoise samples. As mentioned above, each of the latter power valuesrepresents the average power contained in a 200 millisecond window ofnoise signal samples. More particularly, the program at block 539determines the average power level for the aforementioned groupsobtained from the F direction path by dividing the contents ofaccumulator FDSPEECH by FDCTR. The program then converts the resultingvalue to dBm and outputs that result to lead 103 (FIG. 1) as the speechmeasurement for F direction. The program then proceeds to block 540where it similarly generates a speech measurement for E direction andoutputs the result to lead 103. The program then proceeds to block 541where it converts FDNSMIN to dBrnC, which is dB referenced to a Picowattusing C-message weighting, and outputs that result to lead 103 as thenoise measurement for the F direction. The program then proceeds toblock 542 where it determines in a similar manner the noise measurementfor the E direction and then outputs that result to lead 103. Theprogram then exits via block 543.

In an alternative embodiment of the present invention, rather thanfinding the minimum of exactly i average values of 100 FDVALs and EDVALsand then ceasing to accumulate such values, the program operates suchthat it need only accumulate a predetermined minimum number of averagevalues of 100 FDVALs and EDVALs, before setting the NSDONE flag equal toone. However, in this alternative embodiment, the program will continueto accumulate average values of FDVALs and EDVALs, even after theminimum number have been accumulated, until the speech levelmeasurements are completely done, i.e., until FDDONE and EDDONE areequal to one. In all other respects, the program functions in anidentical manner.

The foregoing is merely illustrative of the principles of the invention.Those skilled in the art will be able to devise numerous arrangements,which, although not explicitly shown or described herein, neverthelessembody those principles that are within the spirit and scope of theinvention.

What is claimed is:
 1. An apparatus for separately measuring the levelof noise signals and the level of speech signals when they are presenton a transmission path, comprising:(a) first means for determiningindividual average noise power levels for respective succeeding groupsof said noise signals present within a predetermined period of time onsaid path and for determining an average noise power level for saidgroups as a function of said individual average noise power levels; (b)second means for determining individual average speech power levels forrespective groups of said speech signals present within saidpredetermined period of time and for determining an average speech powerlevel for said groups; (c) means for storing as the noise level of saidtransmission path the minimum value of said average noise power levels;and (d) means, operative when a predetermined number of such averagenoise power levels have been determined, for converting the minimumvalue based on C-message weighting and outputting the converted value.2. The apparatus set forth in claim 1, wherein said first means fordetermining includes means, operative in the event that a current one ofsaid individual average noise power levels is indicative of speechsignals, for then discarding those of said individual average noisepower levels determined prior to said current one of said individualaverage noise power levels.
 3. The apparatus set forth in claim 1,further comprising:means operative when a predetermined number of saidindividual average speech power levels have been determined foroutputting to an output terminal said average speech power level as thelevel of speech on said transmission path.
 4. The apparatus set forth inclaim 3, wherein said second means for determining further includesmeans for classifying a signal present on said transmission path as oneof said speech signals if the level of the signal is at least equal to apredetermined threshold, and further comprising:means responsive to saidsignal being classified as one of said speech signals for classifying apredetermined number of succeeding signals present on said transmissionpath as said speech signals even though the levels of said succeedingsignals are below said predetermined threshold and for including saidsucceeding speech signals in a respective one of said groups of speechsignals.
 5. The apparatus set forth in claim 3, wherein saidtransmission path comprises two oppositely directed transmission paths,and wherein said first and second means for determining operate on eachone of said oppositely directed transmission paths.
 6. The apparatus setforth in claim 5, wherein said second means for determining furtherincludes means, operative when said speech signals are presentsimultaneously on respective ones of said oppositely directedtransmission paths, for discarding the average speech power levelsdetermined for those of said signals that represent an echo.
 7. Anapparatus for measuring the level of noise and speech signals on anin-service transmission path, comprising:(a) means for receiving thelevels of respective ones of said noise signals as they appear insuccession within a predetermined period of time on said transmissionpath; (b) means for forming said levels into respective groups of levelsas said levels are received and for generating an average noise powerlevel for each of said groups; (c) means for accumulating each saidaverage noise power level that is generated within a predeterminedperiod of time and for generating an average noise power level for saidaccumulation; (d) means for determining individual average power levelsfor respective groups of speech signals that are present on saidtransmission path; (e) means for outputting to an output terminal avalue representing the minimum of said predetermined number of averagepower levels; and (f) means, operative when a predetermined number ofaverage power levels have been generated for a like number of suchaccumulations, for converting the minimum value based on C-messageweighting and outputting the converted minimum value.
 8. The apparatusset forth in claim 7, wherein said means for forming and generatingincludes means, operative in the event that the average noise powerlevels generated for a current one of said groups is indicative ofspeech signals, for discarding the average noise power level generatedfor those of said groups that were received prior to said current one ofsaid groups.
 9. The apparatus set forth in claim 7, furthercomprising:means operative when a predetermined number of saidindividual average speech power levels have been generated foroutputting a power level indicative of the level of speech signals onsaid transmission path, in which said power level is generated as afunction of the individual average speech power levels generated forsaid groups of speech signals.
 10. The apparatus set forth in claim 9,wherein said means for determining further includes means forclassifying a signal present on said transmission path as a speechsignal if the level of that signal at least equals a predeterminedthreshold, and further comprising:means responsive to a signal beingclassified as a speech signal for then classifying a predeterminednumber of succeeding signals present on said transmission path as speechsignals even though the levels of those signals are below saidpredetermined threshold and for including such speech signals inrespective ones of said groups of speech signals.
 11. The apparatus setforth in claim 9, wherein said transmission path comprises twooppositely directed transmission paths, and wherein said means forforming and said means for determining operate on respective groups ofsignals when they are present on respective ones of said oppositelydirected transmission paths.
 12. The apparatus set forth in claim 11,wherein said means for determining further includes means, operativewhen ones of said groups of signals are present simultaneously onrespective ones of said oppositely directed transmission paths, fordiscarding the average speech power levels determined for those of saidones of said groups of signals that represent an echo.
 13. An apparatusfor separately measuring the levels of noise and speech signals whenthey are present on a transmission path, comprising:(a) means foraccumulating the levels of said noise signals for each of a successionof windows, each of said windows representing a predetermined period oftime; (b) means, responsive to the presence of a speech signal on saidtransmission path during a current one of said windows, for discardingthe associated accumulation; (c) means for separately accumulating thelevels of said speech signals; and (d) means for calculating a minimumof the accumulation obtained for each of said windows, for convertingsaid minimum based on C-message weighting, and for outputting saidconverted minimum to an output terminal as the level of noise signalspresent on said transmission path.
 14. The apparatus set forth in claim13, wherein said means for separately accumulating includes means forclassifying a signal present on said transmission path as a speechsignal if the level of the signal at least equals a predeterminedthreshold, and further comprising:means, responsive to a signal beingclassified as a speech signal, for classifying a predetermined number ofsucceeding signals present on said transmission path as speech signalseven though the levels of those signals are below said predeterminedthreshold.
 15. The apparatus set forth in claim 14, wherein saidtransmission path comprises two oppositely directed transmission paths,and wherein said means for accumulating includes means for accumulatingthe levels of signals when they are present on respective ones of saidoppositely directed transmission paths.
 16. The apparatus set forth inclaim 15, wherein said means for separately accumulating includes means,operative when ones of said signals are present simultaneously onrespective ones of said oppositely directed transmission paths, fordiscarding the accumulation determined for those signals that representan echo.
 17. A method for measuring the level of noise and speechsignals when they are present on a transmission path, comprising thesteps of:(a) accumulating the levels of said noise signals for each of asuccession of windows, each of said windows representing a predeterminedperiod of time; (b) responding to the presence of a speech signal onsaid transmission path during a current one of said windows bydiscarding the associated accumulation; (c) separately accumulating thelevels of said speech signals; and (d) calculating a minimum of theaccumulation obtained for each of said windows, converting said minimumbased on C-message weighting, and then outputting said minimum to anoutput terminal as the level of noise signals present on saidtransmission path.
 18. A method for measuring the level of noise andspeech signals when they are present on a transmission path, said methodcomprising the steps of:(a) determining individual average noise powerlevels for respective succeeding groups of said noise signals occurringwithin a predetermined period of time and determining an average noisepower level for said groups as a function of said individual averagenoise power levels; (b) calculating the average noise power level forsaid groups and average noise power levels for respective other ones ofsaid groups; (c) determining individual average speech power levels forrespective groups of speech signals when they are present on saidtransmission path; (d) storing as the noise level of said transmissionpath the minimum value of said average power levels; and (e) convertingthe minimum value based on C-message weighting and outputting theconverted minimum value when a predetermined number of such averagepower levels have been calculated.
 19. A method according to claim 18,further comprising the step of discarding, in the event that a currentone of said individual average noise power levels is indicative ofspeech signals, those of said individual average power levels determinedprior to said current one of said individual ones of said power levels.20. A method according to claim 18, further comprising the stepof:outputting, when a predetermined number of said individual averagespeech power levels have been determined, the average value of saidpredetermined number of average speech power levels as the level ofspeech on said transmission path.
 21. A method according to claim 20,wherein a signal present on said transmission path is classified as aspeech signal if a level of said signal is at least equal to apredetermined threshold.
 22. A method according to claim 21, wherein apredetermined number of signals succeeding a signal classified as aspeech signal are classified as speech signals even though they have alevel which is below said predetermined threshold, and wherein saidpredetermined number of succeeding signals are included in respectiveone of said groups of speech signals.
 23. A method according to claim18, wherein said transmission path comprises two oppositely directedtransmission paths, and wherein said determining steps includedetermining average noise and speech power levels for respective groupsof signals when they are present on respective ones of said oppositelydirected transmission paths.
 24. A method according to claim 23, furthercomprising the step of discarding, when ones of said groups of signalsare present simultaneously on respective ones of said oppositelydirected transmission paths, the average speech power levels determinedfor those ones of said groups of signals that represent an echo.